Ground-source heating and cooling, if designed and installed correctly, can provide more energy-efficient heating and cooling than conventional systems such as gas-fired boilers and air-cooled chillers. However, the cost and complexity of installing a system, much of which will be buried underground, means that care and attention are required during the design, construction and commissioning stages.
Heat pumps make use of the relatively constant year-round ground temperature to provide heating, while ground-source heating and cooling systems (GSHCS) transfer heat to and from the ground and are thus able to provide heating and cooling in one integrated system. These systems can be installed in either open-loop or closed-loop forms.
Closed-loop pipework is often buried horizontally at a depth of a few metres over a wide area. The loops are commonly referred to as slinkies, thanks to their resemblance to the toy, and where there is insufficient area the pipes can be installed vertically in the ground as boreholes.
Pipes can also be installed within the structural piles of the building; in this case closed loops are placed in a steel-reinforced cage and then encased in concrete to form the pile. The structural pile design must take into account the thermal loads imposed by the GSHCS.
The shallower depth of the structural pile will provide less capacity than the deeper vertical boreholes, but can be a lower-cost option as the only uplift is the addition of the pipe to the structural pile. This is referred to as a vertical closed-loop thermal pile. Deeper piles, around 25m down, will be more economic than shallower ones, as increasing the depth will provide greater heat transfer.
Ground-source system function can be classified as follows.
An unfused pipe joint excavated following a system leak. In this case, the electrofusion joint was not completed correctly IMAGE @ NEVILLE RYE
It is important to get a number of things right during the design stage of vertical closed-loop borehole systems.
As the key parts of any ground-source system are buried in the ground, and in some instances under the building the quality of the installation must be carefully controlled. Rectification work can be costly and in some instances impossible.
The following measures are advised.
It is important to ensure independent testing and inspection of all interconnecting pipework before placement of backfill material, plus third-party inspection of borehole placement. Do sign off all borehole and system testing records.
For closed-loop systems, the antifreeze content must be maintained in accordance with the design. Monitoring the system for leaks is also important as any water in the make-up or drop in pressure could signal a leak. For open-loop systems, building owners will need to comply with the Environment Agency licence requirements.
Neville Rye is a director at WSP firstname.lastname@example.org
Related competencies include: Construction technology and environmental services